This invention relates to a process for manufacturing a semiconductor memory device, and more particularly to improvements in manufacturing a semiconductor dynamic memory cell.
Conventional MOS dynamic random access memory devices have comprised in many cases, a multitude of n-channel dynamic memory cells each including a single transfer transistor and a single memory capacitor disposed on one surface of a p type semiconductor substrate. In recent years such memory cells have had their area reduced because of the need for high density integration. This reduction in area of the memory cell has caused a decrease in the capacitance thereof. In order to minimize or substantially eliminate the decrease in capacitance of the memory capacitor, there has been proposed a measure to thin an electrically insulating oxide film disposed in the memory capacitor and therefore lower the source voltage. A decrease in source voltage has been inevitably attended with a reduction in the amount of electric charge accumulated in the memory capacitor.
Under these circumstances, if the p type semiconductor substrate is irradiated with .alpha. rays originating from a radioactive substance included in a package for the memory cells, then a malfunction has been actualized in the memory cell. That is, those .alpha. rays cause a multitude of electron-hole pairs to be formed within the substrate and diffused into the substrate. Holes thus formed are absorbed by the p type semiconductor substrate but electrons thus formed reach the memory capacitor and/or a bit line connected to the memory capacitor through the transfer transistor. Thus, the so-called soft errors have been actualized. More specifically, the electrons are collected in the memory capacitor and/or in the bit line to erase data respectively stored in the memory capacitor and/or in the bit line.
In order to avoid such soft errors, there has been proposed a measure to use semiconductor substrates which are low in resistivity. This measure has adversely affected the resulting memory cells in that the transfer transistor increases in threshold voltage and/or in junction capacitance between the semiconductor substrate and each of the source and drain regions of the transistor. Also, a semiconductor substrate has determined the threshold voltage of the transfer transmitter, thereby resulting in a lack of controllability.
Furthermore it has been alreadily proposed to grow a high resistivity semiconductor layer on such a low resistivity semiconductor substrate according to an epitaxial growth technique and then dispose a desired semiconductor element or elements on the high resistivity semiconductor layer. However, during the epitaxial growth, an impurity from the low resistivity semiconductor substrate is diffused into the semiconductor layer being grown so as to change a resistivity of the grown layer. This has resulted in a fear that the completed transistor has deteriorated characteristics.
Accordingly, it is an object of the present invention to provide a new and improved semiconductor memory device which is resistant to .alpha. rays without the characteristics of its transfer transistor being adversely affected.